It is also well known that during the manufacture of printed circuit boards, organic material can be removed from drilled holes and from the surface of a printed circuit board using plasma gas techniques. Normally in such plasma processes a mixture of oxygen and carbon tetrafluoride is used as the process gas. It is known that the etch rate in such plasma systems depend on a variety of parameters such as the amount of RF power, the chamber pressure, the feed gas flow rate, the feed gas composition, etc.
It is also known that the etch rate of organic polymers, photoresists and semi-conductor materials can be increased by alternating pure oxygen plasma with a CF4/O2 mixture (disclosure EN8850106, application serial number 718,246, filed Apr. 1, 1985, abandoned). Furthermore, it has been found that fast etching can be achieved by using several pulses of pure oxygen gas in conjunction with a CF4/O2 plasma (see RESEARCH DISCLOSURE, October 1986, Number 270, page 27025).
One factor which inhibits the efficacy of plasma clean operations is that during the cleaning operation, a monolayer of polymerized fluorocarbon sometimes forms on the surface of the material being cleaned. This layer of polymerized fluorocarbon has teflon like properties and it inhibits further cleaning by the plasma.
Other prior art techniques (such as that shown in U.S. Pat. No. 4,522,681) use a carrier gas along with the process gases. The process gases are gases such as oxygen and carbon tetrafluoride. The carrier gases are inert gases such as Helium and Argon. In such prior art the percentage of inert gas used is relatively high; that is, the percentage of inert gas is generally above thirty percent. Furthermore, since the inert gas is only used as a carrier, either a low mass gas such as Helium or a high mass gas such as Argon can be used.